Large capacity generators, which outputs large current, have large electromagnetic force and heat generation in their armature coils. To cope with this, an armature coil is constructed of a plurality of parallel circuits to reduce the current per coil and suppress the electromagnetic force and the temperature raise.
However, when the number of parallel circuits is not equal to a submultiple of the number of poles, the current of the parallel circuits is biased, producing a current circulating between the parallel circuits. This circulating current will increase a loss in an armature coil and raise the temperature of the coil, leading to a problem of degradation in efficiency and damage to coil insulation.
Here, first, a coil connection method for two-pole, two-parallel circuit configurations used mainly in large generators will be compared with a coil connection method for two-pole, four-parallel circuit configurations obtained by increasing the number of parallel circuits to four. Then a description will be given to the principle of the generation of a circulating current between circuits caused by the increase in the number of parallel circuits.
FIG. 15 illustrates an axial section of the stator of a generator. In the generator shown in FIG. 15, when the number of poles of the rotor 14 is two, the number of slots is 72 and, therefore, the number of slots per phase and per pole is 12. As shown in FIG. 15, the stator core 11 formed of magnetic steel sheets houses armature coils on the inner radius side. For this purpose, slots 5 extending in the axial direction are formed at predetermined intervals in the circumferential direction in the stator core 11. Teeth 4 are present between the slots 5 in the circumferential direction. Two armature coils are housed in each of the slots 5 on the upper and lower sides in the radial direction (inner and outer radius sides). The coils housed on the inner radius side are designated as top coil 12 and the coils housed on the outer radius side are designated as bottom coil 3.
FIG. 16 shows the stator illustrated in FIG. 15, developed in the circumferential direction. This drawing shows only the armature coils in the U phase of three phases, U, V, and W. The coordinate axis θ indicates the circumferential direction and Z indicates the axial direction. The coordinate axes in FIG. 15 and FIG. 16 indicate the same orientations.
As shown in FIG. 16, the top coils 12 and the bottom coils 3 are housed in the stator core 11 and cyclically arranged in the circumferential direction. Since the number of slots per phase and per pole is 12, the number of top coils 12 is 12 and the number of bottom coils 3 is 12 in each pole and, therefore, 24 coils are each present in two poles. Here, a coil group of the top coils 12 and bottom coils 3 arranged for one pole will be defined as phase belt. A phase belt 6 is equivalent to the coil group indicated by the reference character 6 in FIG. 16.
Here, U1 and U2 will be taken for two parallel circuits. In case of a two-pole, two-parallel circuit configuration, the currents of the parallel circuits are balanced by taking U1 as one phase belt 6 and U2 as another phase belt 6. As a result, a circulating current is not produced between the parallel circuits. However, when the number of the parallel circuits is increased to four for current reduction, it is necessary to place two parallel circuits U1 and U2 in one phase belt 6. In this case, the currents of U1 and U2 are brought out of balance and a circulating current is produced between the parallel circuits.
One of methods for suppressing a circulating current is changing the combination of coil connections. For example, U.S. Pat. No. 2,778,962 discloses a connection method for reducing the circulating current. FIG. 17 illustrates this method. FIG. 17 shows only one phase belt and another phase belt is identical to this phase belt. In FIG. 17, the stator core 11 is not shown.
In the method disclosed in U.S. Pat. No. 2,778,962, shown in FIG. 17, the coils are arranged by changing the combination of coil connections in consideration of the voltage balance when a parallel circuit is opened. Imbalance of the currents is thereby suppressed. However, seven jumper connections 8 for changing the combination of coil connections are used per phase belt at the axial end on the connection side where terminals 7 for taking out output are present. With this configuration, the number of phase belts is 6 for the three phases and thus 42 jumper connections 8 are required. This configuration increases the number of parts of the jumper connections 8 and complicates the joints of the jumper connections 8 and causes a problem of degraded workability.
JP 5,193,557 discloses a connection method for reducing the number of jumper connections 8. FIG. 18 illustrates this method. In the method disclosed in JP 5,193,557, shown in FIG. 18, contrivance is given to the relative positions of the top coils 12 and the bottom coils 3 in one of the phase belts, the relative positions defined by counting the positions of the coils in the direction away from the pole center. The number of the jumper connections 8 is thereby suppressed to 6 per phase belt and to 36 for the three phases. The number of the jumper connections 8 in the method disclosed in JP 5, 193, 557 is smaller than that in U.S. Pat. No. 2,778,962. However, a configuration in which the number of the jumper connections 8 is further smaller is desired in consideration of workability.
In the conventional technologies disclosed in U.S. Pat. No. 2,778,962 and JP 5,193,557, the number of the jumper connections 8 is large. This has been one of the causes that degrade workability and reliability in terms of the maintenance of the insulating strength of joints. Meanwhile, a configuration in which a jumper connection 8 is not used at all increases a circulating current between circuits and windings may be burnt because of resulting excessive heating.